Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages The Stiffness of Rubber Contraction experiment
The Stiffness of Rubber AimsIntroductionTheory of rubber conformationThermodynamics - the entropy springEntropy derivationContraction of rubberContraction experimentVerificationBiaxial tensionBalloon experimentSummaryQuestionsGoing further
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Contraction experiment

The theory predicts that the stiffness of rubber is proportional to the temperature.

\[\sigma = \frac{{kTN}}{{{V_0}}}\left( {{\lambda _3} - \frac{1}{{\lambda _3^2}}} \right)\]

The result of this is that, if the rubber is extended under a fixed load, it is likely to contract when it is heated (even after allowance is made for the thermal expansion).

This can be observed using the following apparatus:

Diagram of apparatus

Diagram of apparatus

Photograph of apparatus

Photograph of apparatus
(Click on image to view larger version)

In the demonstration, a rubber strip is suspended inside a vertical Perspex tube, alongside a metre rule. The rubber strip is stretched by attaching a small load to the bottom end. The rubber is then heated using a hair dryer directed into the top end of the tube. A thermocouple is positioned inside the tube and connected to a digital meter that gives the temperature in degrees Celsius (which must be converted to K for use in the equations).

In order to verify the theoretical explanation, you will need to make five observations from the demonstration:

  • initial unloaded length of rubber strip (L0)
  • loaded but unheated length of the rubber strip (L1)
  • initial temperature (T1)
  • loaded heated length of the rubber strip (L2)
  • final temperature (T2)

Video of the contraction demonstration

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